1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus for effecting color image forming by the utilization of an electrophotographic process.
2. Description of Related Art
As an image forming apparatus for forming a color image, for example, an image forming apparatus for successively superimposing and transferring toner images of respective colors formed on a photosensitive drum which is an image bearing member onto a transferring material such as paper held on a transferring drum (transferring film) has been put into practical use.
In such an image forming apparatus, an electrostatic latent image formed on a photosensitive drum on the basis of an inputted image signal is developed by a toner of a first color (e.g. cyan) to thereby form a toner image, and this toner image is transferred to a transferring material such as paper held on a transferring drum (transferring film). This transferring step is likewise executed for toners of the other three colors, i.e., magenta, yellow and black, and toner images of four colors are successively superimposed and transferred onto the transferring material, whereby a color image can be obtained.
In the latest image forming apparatus of an electrophotographic type using a digital image signal, the latent image is formed by dots of predetermined potential gathered on the surface of a latent image bearing member, i.e., a so-called photosensitive member, and a solid portion, a halftone portion and a line portion are expressed by changing dot density.
This method, however, is liable to give rise to the problem that it is difficult for toner particles to faithfully adhere to the dots and the toner particles become protruded from the dots, and the gradation of a toner image corresponding to the ratio of dot density between the black portion and white portion of a digital latent image cannot be obtained.
Further, when the dot size is to be made small to thereby improve resolution in order to improve the quality of image, the reproducibility of the latent image formed by minute dots becomes more difficult, and this leads to the tendency that the resultant image becomes an image inferior in resolution and particularly the gradation of a hilight portion and lacking in sharpness. Also, the disturbance of irregular dots is felt as a sense of granuation, and becomes a factor which deteriorates the quality of image of the hilight portion.
Such disturbance does not occur to ink jet and printing, and particularly the greatest problems peculiar thereto are the fact that it is an unforeseeable unstable factor of the quality of image, and a low-frequency noise macroscopically caused by a toner image being formed with numerous minute toner particles having a particle diameter of 5 to 10 μm being distributed at random in the outlines of the dots.
When an electrophotographic image is magnified by a loupe or the like and observed therethrough, it will be seen that in the case of electrophotography, a dot, even if called so, is not of a smooth configuration like ink jet, but is formed by numerous minute toner particles having a particle diameter of 5 to 10 μm being distributed at random in the outline of the dot. Further, the finish of dots is not uniform, but there are dots low in toner density forming the dots or high in such toner density (fluctuation in the toner density forming the dot), dots small in dot area and large in dot area (fluctuation in dot area), and dots not only of a circular shape but also of an oval shape (fluctuation in dot shape), and they are not just alike. The unevenness of these factors is substantially random and includes considerable low frequency components. As a result, it is a visible cause of noise.
It is the difference between the toner density and the density of paper that makes this noise conspicuous. As compared particularly with ink jet, electrophotography is subjected to the influence of optical dot gain by the distribution of numberless minute toner particles.
The main cause of the above-described phenomenon is that in the electrophotographic process, minute toner particles are used to form dots. Also, other causes promoting the phenomenon include the unsharpening of dot data in the process from a latent image to transferring through developing in the electrophotographic process, the irregular scattering of the toner attributable to the values of physical properties (electrical resistance and surface roughness) of copy paper, etc., and a phenomenon attributable to an adhering force in the developing process which will hereinafter be described.
The adhering force (chiefly the reflection force of the toner to a developer carrying member) between the toner and a developing sleeve in the case of a monocomponent developer, and between the toner and the carrier in the case of a two-component developer is strong, while on the other hand, the distribution of the charging amount of the toner is uneven and therefore, when these are to be stripped off by a developing bias and be made to fly to a photosensitive drum, there occurs unstable image forming in which it is easy for the toner at one place to fly and it is difficult for the toner at another place to fly, and unevenness occurs to dot forming.
On the other hand, a gradation ink process in the ink jet process disclosed in Japanese Patent Application Laid-Open No. 58-39468 is free from such problems peculiar to electrophotography as noted above because the ink jet process itself is simple and moreover, the performance of exclusive paper supporting the present high quality of image is excellent.
Therefore, it has been found that as compared with the effect of the gradation ink used in an ink jet printer or the like for the improvement in graininess, in the electrophotographic process, the effect of the use of a light-colored toner to the aforementioned observable low-frequency noize attributable to the “fluctuation in the toner density forming dots”, the “fluctuation in dot area” and the “fluctuation in dot shape” is more superb than ink jet.
Moreover, in the point that the optical dot gain which has posed no problem in the ink jet process has been a great hindrance to aiming at a high quality of image in the electrophotographic process using numberless minute toner particles, the introduction of the light-colored toner into the electrophotographic process has brought about great progress.
Accordingly, with the view of solving the above-noted problems, there has been proposed a method of forming an image by using a light-colored toner (hypochromatic toner) for a hilight portion and a dark-colored toner (hyperchromatic toner) for a solid portion. For example, in Japanese Patent Application Laid-Open No. H11-84764 and Japanese Patent Application Laid-Open No.2000-305339, there is proposed an image forming method of forming an image by combining a plurality of toners differing in density from one another. In Japanese Patent Application Laid-Open No.2000-347476, there is proposed an image forming apparatus in which a hypochromatic toner having maximum reflection density equal to or less than a half of the maximum reflection density of a hyperchromatic toner is combined with the hyperchromatic toner. In Japanese Patent Application Laid-Open No. 2000-231279, there is proposed an image forming apparatus in which a hyperchromatic toner of which the image density when the amount of toner on a transferring material is 0.5 mg/cm2 is 1.0 or greater and a hypochromatic toner of which the image density in the above-mentioned case is less than 1.0 are combined together. In Japanese Patent Application Laid-Open No. 2001-290319, there is proposed an image forming apparatus in which a hyperchromatic toner and a hypochromatic toner between which the gradient ratio of recording density is between 0.2 to 0.5 are combined together.
In the case of the prior art as described above, however, there has arisen the following problem.
That is, according to my studies, by the hypochromatic toner being used, the gradation property and granular feeling in a low density area constituted by the hyperchromatic toner alone are improved, there has arisen the problem that the granular feeling in a medium density area wherein the hyperchromatic toner and the hypochromatic toner are mixedly present rather becomes remarkable.
The cause of this is that the state in which a slight amount of hyperchromatic toner is present in the hypochromatic toner is an image which is very unstable in process conditions, but is visually very sensitive.
An ink jet printer using the existing six-color (gradation) ink has solved this problem by finely controlling the discharge amount of the ink, but in an electrophotographic apparatus, this instability has been a great hindrance when a gradation system is adopted.
Accordingly, to solve this problem, the fineness or stabilizations of the image output of the hyperchromatic toner must first be controlled more severely than before.